In so-called embedded-servo disk drives, head position or address information is placed at various locations on the disk in some known relationship with associated blocks of data. The disk controller uses the position information to determine when it should read or write data. It is critical that writes only occur over data that has been requested to be over-written, and that read data is data from the requested area of the disk.
It is common to use distinct address areas on the disk for the purpose of determining position. Such an address area is commonly referred to as the "header". In a conventional manner, the header may have several sub-fields, such as a head number, sector address, and track address. The head number denotes which of multiple read/write heads in the drive is currently delivering data; the sector address indicates rotational position of the disk with respect to the head; and the track address indicates radial position of the head with respect to the disk.
Typically, when it is desired to read or write data, a microprocessor in the disk drive controller computes what the correct address for the data is. When it reads a header, it compares that address to the address read in the header, and only allows the read or write to proceed if the addresses match. This process is known as "verifying the header".
To verify the header, the minimum information that is required in the header is one copy of the address information. However, using only this bare minimum renders the verification very sensitive to normal errors that occur when reading from the disk. Such errors arise, for example, from media defects or noise. If the single copy were not completely error-free, verification would fail and the data transfer would be unable to proceed. Such operation might be problematic in disk drives with normal, non-zero error rates.
In an alternative scheme, multiple copies of the address information are placed in the header so that a measure of redundancy is achieved. The header can then be verified even if not all copies are perfect, as long as some threshold number of copies match each other. While this scheme substantially improves error tolerance, it does so by increasing the area required for the header, thus decreasing the disk area available for storage of data. This scheme is therefore said to suffer from reduced "format efficiency" of the disk.